Abstract:We have studied the mechanical properties of polymer-carbon nanotube composites. Tensile tests were carried out on free-standing composite films of polyvinyl alcohol and six different types of carbon nanotubes for different nanotube loading levels. Significant increases in Young’s modulus by up to a factor of two were observed in all cases. Theories such as Krenchel’s rule-of-mixtures or the Halpin-Tsai-theory could not explain the relative differences between composites made from different tube types. However, it is possible to show that the reinforcement scales linearly with the total nanotube surface area in the films. In addition, in all cases crystalline coatings around the nanotubes were detected by calorimetry suggesting comparible polymer-nanotube interfaces. Thus, the reinforcement appears to be critically dependent on the polymer-nanotube interfacial interaction as previously suggested. Furthermore, additional polymer-multiwall nanotube composite films were fabricated using polyvinylalcohol and chlorinated polypropylene. As observed previously polyvinylalcohol formed a crystalline coating around the nanotubes, maximising interfacial stress transfer. In the second case the interface was engineered by covalently attaching chlorinated polypropylene chains to the nanotubes, again resulting in large stress transfer. Increases in Young’s modulus, tensile strength and toughness of ´3.7, ´4.3 and ´1.7 respectively were observed for the polyvinylalcohol based materials. Similarily for the polypropylene based composites, increases in Young’s modulus, tensile strength and toughness of ´3.0, ´3.9 and ´4.4 respectively were observed. In addition a model to describe composite strength was derived. This model shows that the tensile strength increases in proportion to the thickness of the interface region. This suggests that composite strength can be optimised by maximising the thickness of the crystalline coating or the thickness of the interfacial volume partially occupied by functional groups.